Assessing the Grazing Activity of Sea Urchins
Sea urchins are herbivorous animals that live on rocky substrate and graze on macroalgae. Pollution and overfishing of their predators, led to massive increase of the sea urchin population across the Mediterranean.
As a consequence, macroalgae forests has tended to disappear, causing cascades effects on other organisms that rely on this source of food and shelter, such as fish and other invertebrates.
This project aims to quantity the effect of sea urchin grazing activity on macroalgae cover and benthic organisms that inhabit these habitats. Through a manipulative experiment, the sea urchin assemblage was removed from experimental patches. The algae growth and fish community recovery has been compared between the experimental areas and the control areas, where the sea urchins remained untouched. Eight paired patches with a total area equal to 100m2 (10m x 10m), with similar environmental conditions, were randomly selected along the littoral rocky shore: the density of the sea urchin guild were evaluated in each patch and preliminary surveys were conducted to assess the initial benthic assemblage.
After a month from the beginning of the experiment, the algae growth was recorded across all the patches considering the following groups: arborescent, foliose (Udotea spp.), corticated (Padina pavonica, Acetabularia spp.), leathery (Cystoseria spp.), calcareous (Halimeda spp.), crustuose (calcareous red encrousting algae) and turf (green filamentous algae, smaller than 2cm). However, the experimental areas underwent the highest recovery in terms of algae biomass and herbivorous fish visits.